1. Field of Invention
The present invention relates generally to cleaning devices for semiconductor wafers. More specifically, this invention relates to a novel roller constructed of a polyvinyl acetal composition having an embedded mandrel. The roller has no outer skin and a small average pore size allowing chemical flow through the roller body without clogging of the sponge pores thereby prolonging its useful life.
2. Background of the Invention
Many industries today require the ability to efficiently clean highly finished surfaces, removing particles and other surface contaminants. Specific industries having articles with highly finished surfaces include, but are not limited to, wafers of semiconductor material, and memory disks.
The presence of foreign material on semiconductor wafers represents a serious problem in the manufacture of integrated circuits on semiconductor wafers. In the manufacturing process, the surfaces of the wafers are initially polished prior to subsequent processing resulting in the deposition of desired circuits. A combination of chemical and mechanical polishing is generally used requiring the introduction of a polishing slurry onto the surface of the semiconductor wafer as it is being polished or cleaned. The slurry may contain fine abrasive particles and incorporate chemical cleaning compositions such as silica (SiO2) and alumina (Al2O3). When the polishing is completed the semiconductor wafers must be cleaned to completely remove residual materials to ensure that the surfaces are ready for subsequent photolithographic processing and other manufacturing steps.
Semiconductor wafers must be cleaned prior to any processing steps used to produce semiconductor devices. The pores of currently used chemical mechanical polishing (CMP) roller brushes quickly become clogged with chemicals and waste, reducing flow and prematurely breaking down during the cleaning process.
CMP roller brushes are used to clean the residual cleaning compositions from the surfaces of the semiconductor wafers. These devices for cleaning wafers generally consist of a cylindrical roller passing over a wafer. Alternatively, the rollers are used in tandem, mounted with essentially zero clearance between their cleaning/polishing surfaces. They are then counter-rotated and the wafer is passed between them to effect cleaning, polishing and/or planarization. Conventional CMP roller arrangements can include tufted nylon or other types of bristles extending from a central core and typically include sponge resins of polyvinyl acetal (PVA). The brushes are fixed horizontally and rotate as the wafer or memory disk is passed between the surfaces of the brushes while the surfaces receive large quantities of cleaning solution or de-ionized water.
Wafers cleaned in this manner have a high defect rate due to particles and other surface contaminants missed or passed over in the cleaning process. This results in breakage of the wafers as well as damage to wafer surfaces.
As noted above, more effective cleaning devices using sponge material have become available whereby a foam brush is mechanically affixed to a core or mandrel. This method is used in lieu of the more cumbersome system of mounting the brush or roller onto a core and then mounting the completed unit onto the CMP equipment. Previously known methods of manufacturing these foam brushes include molding the foam directly onto the external diameter of the core or mandrel or pre-mounting the brush onto the core or mandrel. However, the pores of the sponge material quickly become clogged with waste material and the pliability of the sponge material that is necessary for efficacy also makes them particularly prone to surface ripping. It has not been effective to clean such brushes and they are generally discarded after an undesirable amount of residual cleaning composition has been built up in the pores of the brush. Thus, the sponge brushes are generally more effective but require frequent replacement due to the accelerated wear characteristics inherent in their design. However, since such brushes are relatively costly, the dirty brushes are sometimes utilized beyond the point where they should be replaced resulting in inconsistent levels of cleanliness on the wafers lowering the yield of semiconductor product wafers.
3. Description of the Related Art
U.S. Pat. No. 4,098,728 discloses a polyvinyl acetal (PVA) sponge and a method for making the same. In this method, pore spaces in the sponge are formed by gas bubbles; rather than pore forming chemical additives such as starch/sulfate combinations. Because the sponge disclosed in this patent does not have any starch residue, it has been particularly useful in medical applications in which starch residues can cause a foreign body reaction when in contact with human tissue.
The use of synthetic sponges for cleaning devices is well known. For example, U.S. Pat. No. 4,566,911 discloses a roller scrubbing device using a PVA material for cleaning semiconductor chips having a surface layer of elastic PVA material with an average pore diameter ranging from 10 to 200 microns. U.S. Pat. No. 6,080,092 discloses an industrial sponge roller device with a cylindrical body of PVA material and a plurality of projections of a truncated conical shape extending from an outer surface of the cylindrical body.
Conventional synthetic sponges have a polymer structure with “dead end pockets” formed therein that trap residue and trace amounts of metals and have non-uniform pore sizes causing fluid backup and residue deposit. As the sponge wears, these metals can come out of the sponge in the form of particulate matter. Such particulate matter can damage the surfaces that are to be cleaned. Further, this type of sponge has tiny fibrils in the pores thereof that are a result of spaces between the pore forming chemical additive during a cross-linking reaction. “Cross-linking” is the formation of ester bonding between chains of the two adjacent hydroxyl groups that occur with the reaction of polyvinyl alcohol and aldehyde. This reaction hardens and strengthens the resulting material.
These conventional synthetic sponge rollers are generally manufactured by affixing the foam roller material about a more rigid core or mandrel. However, previously known methods of manufacturing CMP brushes results in insufficient adhesion of the foam brush material to the core or mandrel, resulting in slippage and distortion during use. In operation, the foam brush roller is generally rotated at approximately 500 revolutions per minute with the result that irregular deformation of the sponge material occurs due to impingement against the wafer material and centrifugal forces exerted by rotation. Moreover, axial distortion or twisting of the foam brush occurs due to irregular slippage of the roller about the mandrel. The result is an unacceptable degree of irregularity in the planarization, cleaning, and/or polishing of the wafer surface.